This application claims priority under 35 U.S.C. xc2xa7xc2xa7119 and/or 365 to 9904653-4 filed in Sweden on Dec. 17, 1999; the entire content of which is hereby incorporated by reference.
The present invention relates to methods and apparatus pertaining to microwave chip carriers.
Large and heavy equipment, such as waveguides, mixers, amplifiers, etc., were often earlier used in microwave technology. However, developments in semiconductor technology have enabled microwave functions for frequencies of about 2 GHz and higher frequencies to be implemented at present with components on integrated microwave chips, for instance with the aid of semiconductor processing techniques and materials such as gallium-arsenide (GaAS) and silicon (Si). Microwave chips are used for an increasing number of technical functions in microwave technology, such as amplifiers, mixers, frequency multipliers, etc., and it is not unusual to incorporate several functions in one single microwave chip. The microwave chips are less expensive, smaller and lighter in weight than traditional devices. Consequently, the microwave chip is well suited for use in many modern technical applications, for instance in aircraft electronics, radar, miniantennas, base stations, radio links, and so on.
The microwave chip is constructed around a dielectric substrate that has semiconductor components disposed on its upper surface. The semiconductor components are normally connected with microstrip conductors (or coplanar conductors). The microstrip conductors include signal conductors disposed on the upper side of the substrate, and an electrically conductive element that is adapted to define an earth plane for the signal conductors on the underside of the substrate. The microwave chip is normally very small (about 2-10 (mm)2) and thin (about 0.1 mm).
The microwave chips are normally disposed on a chip carrier, said carrier normally including a carrier element in the form of a piece of material that has context-suitable properties. The microchip is mounted on a surface of the carrier element by means of an electrically conductive bonding substance, such as solder or glue.
There are several reasons for mounting microwave chips on chip carriers. One reason is because the carrier provides mechanical support for the chip. The carrier also enables heat to be conducted from the microwave chip to a cooling sink more effectively, for instance. The carrier also often forms an earth potential for the earth plane of the chip. Normally, the carrier is much larger than the chip, so as to provide space for elements required to connect the chip to peripheral equipment.
Thus, the carrier element will preferably consist of material that will conduct heat and current effectively. Magnet carrying ferrite cylinders are often disposed on the carrier. The thermal expansion of the carrier element will also preferably be adapted to the thermal expansion of the microwave chip mounted on the carrier. Typical carrier element materials are, for instance, copper-tungsten (CuW), copper-molybdenum (CuMo) and aluminium-silicon carbide (AlSiC), these materials being suitable for microwave chips in gallium-arsenide; or aluminium-nitride (AlN), which is suitable for silicon microwave chips.
However, the known technology for mounting microwave chips on chip carriers has certain drawbacks. When the microwave chip is bonded to the carrier elements, the bonding substance forms an impedance which gives rise to an undesirable potential difference (earth fault) between the earth plane of the microwave chip and the earth potential of the carrier element. Furthermore, the materials used in chip carriers are often very expensivexe2x80x94it is not unusual for the chip carrier to cost up to 50% of the cost of the actual microwave chip. This high chip carrier cost has not earlier been considered a significant drawback, as the microwave chips obtain significant advantages in comparison with traditional techniques. The rapid development in technology and the increasing competition within this technical field, however, has meant that the high costs of the chip carriers must now be considered to constitute a burdensome drawback.
The problem mainly addressed by the present invention resides in providing method and means that will enable microwave chips to be mounted on chip carriers in a manner that will reduce an earth fault between the microwave chip and a chip carrier to a minimum.
In brief, this problem is solved in the following manner. The microwave chip is mounted on a chip carrier that includes an electrically and thermally conductive element in which a recess is provided relative to a surface of said element. The microwave chip is bonded or fastened to the surface of the electrically and thermally conductive element by means of a bonding substance that is disposed at least partially in the recess. The chip is positioned so that a chip earth plane will be level with the surface of the electrically and thermally conductive element. The distance between the earth plane and the surface is therefore very small, and consequently no earth fault will occur between the earth plane and said surface.
Thus, an aim of the present invention is to enable microwave chips to be mounted on chip carriers so that no earth fault will occur, this aim being achieved with methods and arrangements according to the invention.
Thus, a main advantage obtained when mounting chips in the aforesaid manner is that the occurrence of earth faults is avoided. Another advantage is that heat dissipation from the microwave chip is made more effective to some extent, due to the increase in the contact surface area between the bonding substance and the electrically and thermally conductive element provided by the recess.
More explicitly, the aforesaid problem is solved in accordance with specific embodiments. For instance, if the electrically and thermally conductive element is comprised of a layer of copper or gold, as is preferred, the bonding substance will be solder or glue, for example.
The present invention also addresses the further problem of providing a chip carrier that can be produced simply and inexpensively and that is also suitable for the aforesaid mounting process.
In brief, this further problem is solved with a chip carrier that includes at least a first surface on which a dielectric layer is disposed such as to contribute in forming a pit on the first surface. An electrically and thermally conductive layer is arranged on the dielectric layer and in the pit such that the electrically and thermally conductive layer will obtain a surface in relation to which there is arranged a recess in connection with the pit.
A further aim of the invention is therefore to provide a chip carrier which is suited to said mounting process and which can be produced readily and inexpensively, wherein the invention also includes a method of producing such a chip carrier.
More explicitly, this further problem is solved by specific embodiments. For example, the carrier element may consist of a sheet of brass, aluminium or material that has similar properties and price. The dielectric layer is comprised, for instance, of photosensitive material that is laminated on the carrier element, wherewith the pit is suitably obtained by providing an opening in the dielectric layer. The opening, or cut-out, may be obtained by treating the dielectric layer photochemically. The electrically and thermally conductive layer may be comprised of copper or gold that has been panel-plated on top of the dielectric layer, for instance.
The main advantages afforded by the chip carrier is that it is suited to the aforesaid mounting process, and that it is relatively inexpensive and easy to manufacture.
The invention will now be described in further detail with reference to exemplifying embodiments thereof and also with reference to the accompanying drawings.